1. Field of the Invention
This invention relates to an optical fibre communications system, and in particular to an optical fibre communications system capable of carrying both broadband signals and interactive signals such as telephony and ISDN.
2. Related Art
In optical transmission systems, the radiation used is not necessarily in the visible region of the electromagnetic spectrum, and so the word "optical" and "light" when used in this specification are not to be interpreted as implying any limitation to the visible spectrum. For example, the wavelengths preferred for transmission through silica optical fibres are in the infra red region of the spectrum, because the low loss minima of silica fibres occur at about 1.3 and 1.5 microns.
Optical transmission systems may be utilised to distribute both interactive services such as telephony and ISDN, and broadband services such as video channels, video telephony and information services such as picture videotext. In general, the primary service, at least as presently measured in terms of subscriber lines, is telephony. Increasingly, however, there is a perceived need for optical transmission systems to be able to carry both interactive services and broadband services.
Various techniques are available for separating different services for transmission over the same lines, for example the transmitted signals may be time, wavelength or sub-carrier frequency multiplexed. Wavelength division multiplexing (WDM), with different services on different wavelengths, would require additional optical transmitters and receivers to be installed wherever an expansion of services and additional channels is required.
Conventional broadband service provision, for example that provided by the cable TV companies, uses amplitude modulated (AM) transmission. Unfortunately, AM transmission is not suitable for passive optical networks (PONs), due to signal-to-noise-ratio limitations and intermodulation distortion imposed by optical transmitters. Although lasers are being developed which offer the prospect of meeting the required performance in these two areas, the optical budget achievable will still be very limited, and optical splitting of the signal will, therefore, be minimal.